In recent years, a rapid development of increasing the density of optical disks has been made and, with this rapid development, recording of a moving picture image with a large capacity has become possible. Moreover, optical disks as a storage medium have excellent properties such as excellent capacitive properties and high speed accessibility, can be provided at low cost, and also is interchangeable. In view of these points, it is highly possible that optical disks will take place tape recording medium.
As for recording methods for optical disks, there are the PPM (pulse point modulation) recording method in which information is indicated on a recording mark and the PWM (pulse width modulation) recording method in which information is indicated on a recording mark edge. At present, the PWM recording method advantageous to increasing density has been used in most cases.
In the PWM recording method, when a recording mark is formed to be large or small depending on recording conditions, the duty ratio of a reproduction signal shifts from a predetermined value, resulting in the generation of asymmetric waveform distortion in the amplitude direction. This phenomenon is called “asymmetry”.
When a disk such as a CD (compact disk) and a DVD (digital versatile disk) in which data is recorded with a discrete mark length and a discrete space length is played and observation is made using an oscilloscope, a waveform of FIG. 1(a) or FIG. 1(b) is obtained. In general, such waveforms are called “eye pattern”. When asymmetry is absent, the waveform of FIG. 1(a) is obtained, and when asymmetry is present, the waveform of FIG. 1(b) is obtained. In this case, a hatched portion having a rhombus shape is called “eye diagram” and a shift of an eye center level with respect to a reproduction waveform center (the center of a waveform) in the vertical direction shows an asymmetry amount. In the PWM recording method, information is held at a recording mark edge, and when asymmetry is present, a shift of a recording edge occurs. Thus, a measure for this problem is needed.
Assume that data reproduction is carried out by a simple binarization of a reproduction signal. For example, when recording modulation in which no direct current (DC) component is used as in a CD, influence of asymmetry can be almost eliminated, for example, by performing feedback control of a binarization level so that the duty ratio after the binarization becomes 50:50. Thus, influence of asymmetry can be substantially eliminated.
FIG. 2 is a block diagram illustrating an example of a conventional binarization circuit. This binarization circuit includes a comparator circuit 100 for binarizing a reproduction signal at a predetermined level, an integrator circuit 101 for integrating a comparator circuit output, a ripple removing filter 102 for removing a ripple of an integrator circuit output, and a buffer circuit 103 for performing feedback of a ripple removing filter output to the comparator circuit 100. In this case, binarization levels appear around the respective eye center levels of the eye diagrams of FIG. 1(a) and FIG. 1(b).
However, as shown in FIG. 3, when aliasing distortion of a reproduction waveform is generated and a large asymmetry is present, a binarization level overlaps aliasing part of the waveform and binarization can not be properly performed with the configuration of FIG. 2. As a result, data is not properly reproduced. Such aliasing distortion of a reproduction waveform is apt to occur when a laser spot diameter is reduced more than necessary on a recording surface or when a high frequency band of the reproduction signal is emphasized by an equalizer or the like.
On the other hand, in a digital reproduction signal processing system using a Viterbi decoder, a reproduction signal is sampled by an analog-to-digital converter (which will be hereinafter referred to as an “AD converter”) and decoded data corresponding to a state transition maximum-likelihood-estimated based on multilevel data obtained through the sampling is output. Ideally, vertical asymmetry of the amplitude direction of the reproduction signal is required.
FIG. 4 is a block diagram illustrating an exemplary block diagram of a conventional signal processing device using the PRML (partial response maximum likelihood) technique. The signal processing device includes an AD converter 104 for analog-to-digital converting a reproduction signal (RS1), a baseline processing circuit 105 for removing a DC fluctuation component from an AD conversion signal (ADCOUT), a PLL circuit 106 for extracting phase error information from a baseline processing signal (BCDT) to generate a clock phase-synchronized with an input reproduction signal, an FIR (finite impulse response) filter 107 for receiving the baseline processing signal as an input and performing waveform equalization, an LMS (least mean square) circuit 108 for adaptively adjusting tap coefficients of the FIR filter 107 so that an equalization error becomes minimum, and a Viterbi decoder 109 for outputting decoded data corresponding to the maximum-likelihood-estimated state transition from an FIR filter output (FIRDT).
With the configuration of FIG. 4 to which the PRML technique is introduced, it is possible to largely improve an error rate, compared to data reproduction by the simple binarization shown in FIG. 2, and improve the performance of the signal processing device. However, the PRML technique is designed based on an ideal waveform exhibiting vertical symmetry in the amplitude direction, and when strong asymmetry occurs in a reproduction signal and the vertical symmetry in the amplitude direction is largely lost, the Viterbi decoder 109 does not properly operate.
Note that as a conventional signal processing method, a method in which based on a reproduction signal value and operation results of Viterbi decoding, an amplitude reference value used as a reference value when a value for brantimetric is updated for each clock, thereby performing a predetermined calculation based on the amplitude reference value (see Japanese Laid-Open Publication No. 10-320920) is known.
Moreover, a method in which a reproduction signal output from a waveform equalization circuit (equalizer) is sliced at a slice level and the slice level is used as information for the asymmetry amount and a method in which maximum and minimum values of a reproduction signal are detected and a reproduction signal amplitude is made to be constant by amplitude detection to reduce a detection error (see Japanese Laid-Open Publication No. 2001-250334) are known.
However, in general, the amplitude of a reproduction signal from a pickup for reading recorded information is very small. Therefore, it is necessary to acquire a large gain so that a desired signal amplitude is achieved in an amplifier in a subsequent stage, but amplification by DC coupling is difficult because of a constraint of a dynamic range of the subsequent stage amplifier. Therefore, a method in which a DC component is removed by capacitive coupling and then a large amplification is performed is used in many cases. If the DC component is removed, a center level of an eye diagram substantially corresponds to a reference voltage level (i.e., a ground level GND in this case). When asymmetry is absent, a waveform shown in FIG. 5(a) is obtained, and when asymmetry is present, a waveform shown in FIG. 5(b) is obtained. When such a signal is binarized, a binary slice level is located substantially at the center of an eye diagram in each of FIGS. 5(a) and 5(b). Accordingly, the slice levels in FIGS. 5(a) and 5(b) are located around GND and correspond to each other. Thus, the asymmetry amount information can not be obtained from each of the slice levels itself.